Thermal imaging cameras have been around for many years, wherein the sensitivity of the detectors to infrared (IR) radiation allows them to acquire images in darkness and through viewing obscurants such as dust and smoke. Thermal imagers designed primarily for the MWIR and LWIR spectral regions have provided a powerful tool for fire departments, emergency personnel and law enforcement agencies in addition to the military usage. For example, the Law Enforcement Thermographers Association recognizes eleven areas of usage, including search and rescue, fugitive searches, vehicle pursuits, flight safety, marine and ground surveillance, perimeter surveillance, officer safety, structure (building) profiles, disturbed surface scenarios (tracks), environmental law enforcement, and hidden compartments in vehicles.
Light absorption properties of the atmosphere define several frequency bands that are favorable to the transmission of light without undue absorption. Generally, these spectral ranges are defined as the visible band, the near infrared band, the short wave infrared (SWIR) band, the medium wave infrared (MWIR) band, and the long wave infrared (LWIR) band. The LWIR band is dominated by emitted light or thermal energy and spans roughly 8-14 μm with approximately 100% transmission in the 9-12 μm band, providing excellent visibility for most terrestrial objects. The MWIR band has both reflected and emitted radiation and spans about 3.3-5.0 μm allowing nearly 100% transmission with the additional benefit of lower ambient background noise. SWIR and near infrared (NIR) span the band from about 0.3-2.5 μm.
Infrared sensors are devices sensitive to radiation in a limited spectral range of infrared radiation, typically from one of the NIR, SWIR, MWIR or LWIR bands. Such sensors have been used for night vision applications for many years. The detector is essentially a transducer that converts IR energy into a measurable form. The detected energy can be transformed into imagery showing the differences between objects that might otherwise be obscured, wherein objects with differing levels of infrared emissions are discernable.
In general, there are two methods of infrared detection, namely energy detection and photon detection. Energy detection, sometime called thermal detectors, respond to temperature changes and have broad and generally uniform spectral responses with somewhat lower sensitivities and relatively slow response times. The thermal detectors can usually operate at or near room temperature. There a number of known energy detectors, including thermocouples/thermopiles, pyroelectric detectors, ferroelectric detectors, thermistors/bolometers/microbolometers and microcantilevers to name a few.
The photon detection or photodetectors generate free electrical carriers based on the incident infrared radiation and have more limited spectral responses, higher peak sensitivities, and relatively faster response times. The photon detection devices are ideal for fast scanning and imaging application while the uncooled energy detector devices are usually used for portable spot measurements. Other photon detectors include photovoltaic intrinsic detectors, photoconductive intrinsic detectors, extrinsic detectors, photo-emissive detectors, and quantum well photodetectors. There have also been attempts at two-color or dual-band photon detector arrays. These dual-band arrays can provide simultaneous detection in two wavelength bands, but the trade-offs in the design process generally limit the capabilities.
The infrared photon detectors are typically based on semiconductor materials fabricated with the elements in chemical tables III-V, such as indium, gallium, arsenic, antimony; or based on the elements in tables II-VI such as mercury, cadmium and telluride; or with the table elements IV-VI such as lead, sulfer and selenide. There are a number of permutations/combinations, including the binary compounds GaAs, InSb, PbS, PbSe or others including InGaAs and HgCdTe.
In the state of the art there are infrared photon sensing technologies that use semiconductor materials such as HgCdTe or InSb that require cooling in order to stabilize their infrared sensitivity while increasing the contrast of the acquired images. For example, one infrared device utilizes an LWIR sensor and a display screen to detect and display thermal energy. However, the LWIR sensor requires cryogenic cooling to maintain the sensor at stable conditions and high quantum efficiency. Detector cooling can be accomplished by thermoelectric Peltier cooling, compressed argon, liquid Nitrogen and Stirling-cycle cooling. The cooling adds substantial cost and bulk to the LWIR sensor, thus limiting the applications where cryogenically equipped LWIR sensors may be used. Thus, in general, the infrared sensors that generate signals based on photon absorption are relatively complex, costly, and not highly portable.
State of the art thermal imaging applications typically use uncooled thermal cameras that employ focal plane array (FPA) sensors. These FPA sensors use materials such as amorphous silicon, vanadium oxide (VOx) and barium strontium titanate (BST) to form thermal imager detectors. These FPA devices operate at or near room temperature, a characteristic that reduces system complexity, size, and cost. As the FPA sensors absorb incoming infrared radiation, they detect minute changes in resistance in the case of microbolometers made of VOx or amorphous silicon or changes in capacitance for the BST ferroelectric sensors rather than converting the electromagnetic radiation to electrons.
As known in the art, miniature or microminiature bolometers or microbolometers are used as detector pixel elements in two dimensional arrays of thermal infrared detectors. The two dimensional array of microbolometers converts the infrared energy arriving from a scene of interest into electrical signals that are applied to a readout integrated circuit (ROIC). After amplification and desired signal shaping and processing, the resulting signals can be further processed as desired to provide an image of the scene of interest.
A microbolometer typically includes a polycrystalline semiconductor material, such as vanadium oxide (VOx) or titanium oxide, having an electrical resistivity that varies as a function of temperature. An absorber of infrared, such as Si3N4, is provided in intimate contact with the polycrystalline semiconductor material so that its temperature can be changed as the amount of infrared energy arriving from the scene changes. Preferably, the polycrystalline semiconductor/absorber structure is thermally isolated from the underlying ROIC.
There are numerous design constraints that require tradeoffs in the design process for optimization for a particular application. During the design stage, the fabrication parameters are tailored for specific applications as the design choices affect the imaging properties. For example, automatic contrast and brightness controls minimize the need for user adjustments but sacrifice image optimization. The operating environment, the type of object under observation, the size, cost and quality desired are just some of the factors considered during the design stage.
One useful parameter for camera specifications is the noise equivalent temperature difference (NETD), which is a measure of the thermal sensitivity versus temporal noise. Certain uncooled cameras offer NETD values of approximately 100 mK or 0.1 degrees Celsius in the LWIR. While percent differences in NETD from one camera to another may seem significant, other factors contribute to image quality. The level of steady-state or fixed-pattern noise introduced by the detector can degrade image quality, making an imager inappropriate for a specific application, for example. Because this noise is fixed or slowly varying, it has a much greater adverse impact on the image than the transient noise indicated by the value of the NETD.
The modulation transfer function (MTF) serves as another metric for thermal imagers. Within the infrared sensor or detector, the MTF provides a measure of pixel-to-pixel isolation. At the camera level, the MTF also characterizes the quality of the optical system and gives an overall measure of image acuity. Thermal-imaging engineers even combine multiple parameters into a composite figure of merit known as minimum resolvable temperature difference (MRTD). The MRTD essentially measures the level of thermal contrast needed for an observer to distinguish alternating hot and cold bars in a test target. The MRTD is a function of the target spatial frequency (the width and spacing of the bars in the target).
Optics and lens selection is one of the many considerations for IR cameras. For example, the optical elements for MWIR systems tend to use silicon, while LWIR systems generally require germanium. The material choice depends upon many factors including transmission and dispersion. For example, with the appropriate spectral coatings applied to the optics, a single-element lens of silicon can provide about 98% transmission in the 3 to 5 μm range. Likewise, a single-element germanium lens has approximately 95% transmission in the 7 to 14 μm range.
In addition to considering component tradeoffs, engineers developing thermal imagers for law enforcement or military personnel also consider the systems-level design of the imaging units. For detectors in the MWIR and LWIR bands, shield glass lenses are sometimes used to minimize vulnerability to shock damage. The electronics also require protection from environmental conditions such as water and dirt that affects the absorption properties and generally requires structural considerations to minimize apertures and points of entry.
With respect to microbolometers, there are numerous documents describing the state of the art and techniques for fabrication. The fabrication techniques for LWIR microbolometers are well known in the art. There are numerous other texts and patents that depict LWIR microbolometer fabrication processes and variations, as is known to those skilled in the art.
While there have been a number of advancements in the field, the state of the art suffers from a number of disadvantages. The manufacturing processes and end-products employing the uncooled microbolometer arrays are relatively difficult and expensive. There is a need for an uncooled MWIR detector that has not been met. Furthermore, multiple band devices are also need in certain applications. What is needed is an uncooled MWIR microbolometer device and preferably employing familiar processing techniques for manufacturing.